Sputtering of Precursors for Cu2ZnSnS4 Solar Cells and Application of Cadmium Free Buffer Layers

  • Date:
  • Location: Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala
  • Doctoral student: Ericson, Tove
  • About the dissertation
  • Organiser: Fasta tillståndets elektronik
  • Contact person: Ericson, Tove
  • Disputation

The aim of this thesis is to understand the influence of the deposition process and resulting film properties on Cu2ZnSnS4 (CZTS) thin film solar cells. Two main aspects are studied, namely formation of absorber precursors by sputtering, and alternative Cd-free buffer materials with improved band alignment.

Reactive sputtering is used to grow dense and homogeneous precursor films containing all elements needed for CZTS absorbers. The addition of H2S gas to the inert Ar sputter atmosphere leads to a drastic decrease of Zn-deposition rate due to the sulfurization of the target. Sulfurization also leads to instabilities for targets made of CuSn, Cu and Cu2S, while sputtering from CuS gave acceptable process stability.

The H2S/Ar-ratio also affects film morphology and composition. Precursors with sulfur content close to stoichiometric CZTS have a columnar, crystalline structure. Materials analysis suggests a non-equilibrium phase with a cubic structure, where each S atom is randomly surrounded by 2:1:1 Cu:Zn:Sn-atoms, respectively. Substrate heating during sputtering is shown to be important to avoid cracks in the annealed films while stress in the precursor films is not observed to affect the absorber or solar cell quality.

Sputtering from compound targets in Ar-atmosphere yields precursor properties similar to those from reactive sputtering at high H2S/Ar-ratios and both types can be processed into well-performing solar cells.

Additionally, a low temperature treatment of CZTS absorbers in inert atmosphere prior to buffer layer growth is shown to affect the device properties, which indicates that the thermal history of the CZTS absorber is important.

The alternative buffer system ZnO1-xSx is found to yield lower efficiencies than expected, possibly due to inferior interface or buffer quality. The Zn1-xSnxOy (ZTO) buffers instead give better performance than their CdS references. For optimized parameters, the activation energy for recombination coincides with the energy of the photoluminescence peak of the absorber. This can be interpreted as a shift of dominant recombination path from the interface to the CZTS bulk. A well-performing CZTS-ZTO device with antireflective coating yielded an efficiency of 9.0 %, which at the time of publication was the highest value published for a Cd-free pure-sulfide CZTS solar cell.